MP3428 – 17A, 600KHZ, 20V WIDE INPUT RANGE ,SYNCHRONOUS BOOST CONVERTER IN ASMALL 3X4MM QFN PACKAGE

MP3428 Rev. 1.0

www.MonolithicPower.com

12

12/23/2014

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OPERATION

The MP3428 is a 600kHz fixed-frequency, high-

efficiency, wide input range, current-mode

boost converter with optional internal or

external current-sensing configuration for high-

integration and high-power applications (see

Figure 1).

Boost Function

The MP3428 uses constant frequency, peak-

current mode, boost regulation architecture to

regulate the feedback voltage.

At the beginning of each cycle, the N-channel

MOSFET switch Q is turned on, forcing the

inductor current to rise. When floating MODE,

the current at the source of switch Q is

measured externally; then it is converted to a

voltage by the current-sense amplifier. That

voltage is compared to the error voltage on

COMP (which is an amplified version of the

difference between the 1.225V reference

voltage and the feedback voltage). When these

two voltages are equal, the PWM comparator

turns off switch Q. This forces the inductor

current into the output capacitor through the

external rectifier, causing the inductor current to

decrease.

The

peak-inductor

current

is

controlled by the voltage on COMP, which in

turn is controlled by the output voltage. To

satisfy the load, the output voltage is regulated

by

the

inductor

current.

Current

mode

regulation improves transient response and

control loop stability.

VDD Power

MP3428 is powered by VDD. A ceramic

capacitor no less than 2.2μF is required to

decouple VDD. During start-up, VDD power is

regulated by IN. Once the output voltage

exceeds the input voltage, VDD is powered

MP3428 to maintain low Ron and high

efficiency, even with low-input voltage.

Soft-Start (SS)

MP3428 uses one external capacitor on SS to

control SW frequency during start-up. The

operation frequency is initially ¼ of the normal

frequency. As the SS capacitor is charged, the

frequency increases continuously. When the

voltage on SS exceeds ~0.65V, the frequency

switches to a normal frequency. In addition, the

During start-up the COMP voltage

reaches

0.7V quickly, and then rises at the same rate of

inrush current from the input power supply.

SDR and BST Function

The MP3428 generates a synchronous gate-

driving signal that complements the gate driver

of the internal MOSFET. The SDR driver is

N-channel MOSFET (a gate-source threshold

voltage lower than 3V is preferred). In high-

power applications, using a synchronous

rectifier switch improves overall conversion

efficiency. If a synchronous rectifier switch is

not used, float SDR.

The 5V driver power-bootstrap voltage is

powered from OUT. If output voltage is low (or

the duty cycle is too low), BST voltage may not

be regulated to 5V, triggering a BST_UVLO. A

schottky diode from an external 5V source to

BST is recommended, otherwise, the SDR

driver signal may be lost.

Current Sensing Configuration

The MP3428 offers the option of using the

internal circuit or an external resistor to sample

the inductor current. When using the internal

current sense, MODE must be connected

directly to GND before powering on. Meanwhile,

SENSE should be connected to IN. In this

configuration, sensed current is compared to

both the COMP voltage and

the limit peak

current cycle-by-cycle during an overload

condition.

When floating MODE, the inductor current is

sampled by an external sense resistor between

IN and SENSE. Under this configuration,

sensed current is compared with COMP for low-

side switch on/off control. However, the

overload is protected by the average inductor

current. When the sensed current signal

exceeds 54mV, COMP is pulled low to regulate

the boosted current. This causes the MP3428

to enter hiccup mode (after 1.1ms). The

MP3428 re-starts after about 60ms in hiccup